Yikes!!
If I ever spend real money on this game, it will ONLY be for storage... so that I know exactly what I get for my money. No gambling.
How many inventory slots does 100 gems get you? 1 slot, or 5 slots?
Pretty sure each inventory purchase is 5 slots. And an inventory purchase is either 100 gems or 1 mythril.
Spending money in these games is ok IMO as long as you don't approach it with the mindset of wanting a specific item. I did two 11 pulls the other day and I got I think 1 5 star, 4-5 four stars, and a bunch of 3 stars. And even though the 5* was an armor, I still am very happy with the results. I got a ton of III, V, and VI armor which, after upgrading, should allow me to handle all of the V and VI elites. The III stuff I'll just hold onto until they release regular III content. I checked the Japanese wiki and they have a ton of content going all the way through XIII and their difficulty rankings go as high as 120 or so. So I'm definitely going to get some good use out of all of this stuff.
I do feel like the rates on 5* are lower in the US version than in the Japanese version. Feels like 5% to me, whereas the Japanese version explicitly lists it as 10%. So it would be nice if we had parity there, and I don't think I'll pull again until they do that (mostly since I have enough to last me a long time anyway).
Posts like that one on reddit are definitely sad though. If you already know you have a problem with these types of games, best to just stay away.